Group iii nitride crystals, their fabrication method, and method of fabricating bulk group iii nitride crystals in supercritical ammonia

ABSTRACT

In one instance, the invention provides a group III nitride crystal having a first side exposing nitrogen polar c-plane of single crystalline or highly oriented polycrystalline group III nitride and a second side exposing group III polar surface, polycrystalline phase, or amorphous phase of group III nitride. Such structure is useful as a seed crystal for ammonothermal growth of bulk group III nitride crystals. The invention also discloses the method of fabricating such crystal. The invention also discloses the method of fabricating a bulk crystal of group III nitride by ammonothermal method using such crystal.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority to U.S. application Ser.no. 62/086,699 entitled “Group III Nitride Crystals, Their FabricationMethod, and Method of Fabricating Crystals in Supercritical Ammonia,”inventor Tadao Hashimoto, attorney docket SIXPOI-022USPRV1, filed Dec.2, 2014, the contents of which are incorporated by reference in theirentirety herein.

This application is also related to the following U.S. patentapplications:

PCT Utility Patent Application Serial No. US2005/024239, filed on Jul.8, 2005, by Kenji Fujito, Tadao Hashimoto and Shuji Nakamura, entitled“METHOD FOR GROWING GROUP III-NITRIDE CRYSTALS IN SUPERCRITICAL AMMONIAUSING AN AUTOCLAVE,” attorneys' docket number 30794.0129-WO-01(2005-339-1);

U.S. Utility patent application Ser. No. 11/784,339, filed on Apr. 6,2007, by Tadao Hashimoto, Makoto Saito, and Shuji Nakamura, entitled“METHOD FOR GROWING LARGE SURFACE AREA GALLIUM NITRIDE CRYSTALS INSUPERCRITICAL AMMONIA AND LARGE SURFACE AREA GALLIUM NITRIDE CRYSTALS,”attorneys docket number 30794.179-US-U1 (2006-204), which applicationclaims the benefit under 35 U.S.C. Section 119(e) of U.S. ProvisionalPatent Application Ser. No. 60/790,310, filed on Apr. 7, 2006, by TadaoHashimoto, Makoto Saito, and Shuji Nakamura, entitled “A METHOD FORGROWING LARGE SURFACE AREA GALLIUM NITRIDE CRYSTALS IN SUPERCRITICALAMMONIA AND LARGE SURFACE AREA GALLIUM NITRIDE CRYSTALS,” attorneysdocket number 30794.179-US-P1 (2006-204);

U.S. Utility Patent Application Ser. No. 60/973,602, filed on Sep. 19,2007, by Tadao Hashimoto and Shuji Nakamura, entitled “GALLIUM NITRIDEBULK CRYSTALS AND THEIR GROWTH METHOD,” attorneys docket number30794.244-US-P1 (2007-809-1);

U.S. Utility patent application Ser. No. 11/977,661, filed on Oct. 25,2007, by Tadao Hashimoto, entitled “METHOD FOR GROWING GROUP III-NITRIDECRYSTALS IN A MIXTURE OF SUPERCRITICAL AMMONIA AND NITROGEN, AND GROUPIII-NITRIDE CRYSTALS GROWN THEREBY,” attorneys docket number30794.253-US-U1 (2007-774-2);

U.S. Utility Patent Application Ser. No. 61/067,117, filed on Feb. 25,2008, by Tadao Hashimoto, Edward Letts, Masanori Ikari, entitled “METHODFOR PRODUCING GROUP III-NITRIDE WAFERS AND GROUP III-NITRIDE WAFERS,”attorneys docket number 62158-30002.00 or SIXPOI-003;

U.S. Utility Patent Application Ser. No. 61/058,900, filed on Jun. 4,2008, by Edward Letts, Tadao Hashimoto, Masanori Ikari, entitled“METHODS FOR PRODUCING IMPROVED CRYSTALLINITY GROUP III-NITRIDE CRYSTALSFROM INITIAL GROUP III-NITRIDE SEED BY AMMONOTHERMAL GROWTH,” attorneysdocket number 62158-30004.00 or SIXPOI-002;

U.S. Utility Patent Application Ser. No. 61/058,910, filed on Jun. 4,2008, by Tadao Hashimoto, Edward Letts, Masanori Ikari, entitled“HIGH-PRESSURE VESSEL FOR GROWING GROUP III NITRIDE CRYSTALS AND METHODOF GROWING GROUP III NITRIDE CRYSTALS USING HIGH-PRESSURE VESSEL ANDGROUP III NITRIDE CRYSTAL,” attorneys docket number 62158-30005.00 orSIXPOI-005 and issued as U.S. Pat. No. 8,236,237;

U.S. Utility Patent Application Ser. No. 61/131,917, filed on Jun. 12,2008, by Tadao Hashimoto, Masanori Ikari, Edward Letts, entitled “METHODFOR TESTING III-NITRIDE WAFERS AND III-NITRIDE WAFERS WITH TEST DATA,”attorneys docket number 62158-30006.00 or SIXPOI-001;

U.S. Utility Patent Application Ser. No. 61/106,110, filed on Oct. 16,2008, by Tadao Hashimoto, Masanori Ikari, Edward Letts, entitled“REACTOR DESIGN FOR GROWING GROUP III NITRIDE CRYSTALS AND METHOD OFGROWING GROUP III NITRIDE CRYSTALS,” attorneys docket number SIXPOI-004;

U.S. Utility Patent Application Ser. No. 61/694,119, filed on Aug. 28,2012, by Tadao Hashimoto, Edward Letts, Sierra Hoff, entitled “GROUP IIINITRIDE WAFER AND PRODUCTION METHOD,” attorneys docket numberSIXPOI-015;

U.S. Utility Patent Application Ser. No. 61/705,540, filed on Sep. 25,2012, by Tadao Hashimoto, Edward Letts, Sierra Hoff, entitled “METHOD OFGROWING GROUP III NITRIDE CRYSTALS,” attorneys docket number SIXPOI-014;

which applications are incorporated by reference herein in theirentirety as if put forth in full below.

BACKGROUND

1. Field of the Invention

The invention relates to a substrate or a bulk crystal of semiconductormaterial used to produce semiconductor wafers for various devicesincluding optoelectronic devices such as light emitting diodes (LEDs)and laser diodes (LDs), and electronic devices such as transistors. Morespecifically, the invention provides crystals of group III nitride suchas gallium nitride. The invention also provides various methods ofmaking these crystals.

2. Description of the Existing Technology

This document refers to several publications and patents as indicatedwith numbers within brackets, e.g., [x]. Following is a list of thesepublications and patents:

[1] R. Dwilińiski, R. Doradzińiski, J. Garczyńiski, L. Sierzputowski, Y.Kanbara, U.S. Pat. No. 6,656,615.

[2] R. Dwilińiski, R. Doradzińiski, J. Garczyński, L. Sierzputowski, Y.Kanbara, U.S. Pat. No. 7,132,730.

[3] R. Dwiliński, R. Doradziński, J. Garczyński, L. Sierzputowski, Y.Kanbara, U.S. Pat. No. 7,160,388.

[4] K. Fujito, T. Hashimoto, S. Nakamura, International PatentApplication No. PCT/US2005/024239, Wo07008198.

[5] T. Hashimoto, M. Saito, S. Nakamura, International PatentApplication No. PCT/US2007/008743, WO07117689. See also US20070234946,U.S. application Ser. No. 11/784,339 filed April 6, 2007.

[6] D'Evelyn, U.S. Pat. No. 7,078,731.

Each of the references listed in this document is incorporated byreference in its entirety as if put forth in full herein, andparticularly with respect to their description of methods of making andusing group III nitride substrates.

Gallium nitride (GaN) and its related group III nitride alloys are thekey material for various optoelectronic and electronic devices such asLEDs, LDs, microwave power transistors, and solar-blind photo detectors.Currently LEDs are widely used in displays, indicators, generalilluminations, and LDs are used in data storage disk drives. However,the majority of these devices are grown epitaxially on heterogeneoussubstrates, such as sapphire and silicon carbide because GaN substratesare extremely expensive compared to these heteroepitaxial substrates.The heteroepitaxial growth of group III nitride causes highly defectedor even cracked films, which hinder the realization of high-end opticaland electronic devices, such as high-brightness LEDs for generallighting or high-power microwave transistors.

To solve fundamental problems caused by heteroepitaxy, it isindispensable to utilize crystalline group III nitride wafers slicedfrom bulk group III nitride crystal ingots. For the majority of devices,crystalline GaN wafers are favorable because it is relatively easy tocontrol the conductivity of the wafer and GaN wafer will provide thesmallest lattice/thermal mismatch with device layers. However, due tothe high melting point and high nitrogen vapor pressure at elevatedtemperature, it has been difficult to grow GaN crystal ingots.Currently, the majority of commercially available GaN substrates areproduced by a method called hydride vapor phase epitaxy (HVPE). HVPE isone of vapor phase methods, which has difficulty in reducing dislocationdensity less than 10⁵ cm⁻².

To obtain high-quality GaN substrates for which dislocation density isless than 10⁵ cm⁻², various growth methods such as ammonothermal growth,flux growth, high-temperature solution growth have been developed.Ammonothermal method grows group III nitride crystals in supercriticalammonia [1-6]. The flux method and the high-temperature solution growthuse a melt of group III metal.

Recently, high-quality GaN substrates having dislocation density lessthan 10⁵ cm⁻² can be obtained by ammonothermal growth. Since theammonothermal method can produce a true bulk crystal, one can grow oneor more thick crystals and slice them to produce GaN wafers. In theammonothermal growth, bulk crystals of GaN are grown on seed crystals.However, since GaN or other group III nitride crystals do not exist innature, one must fabricate GaN seed crystal with other method.

It is difficult to grow a seed crystal quickly that is suitable for usein ammonothermal bulk growth. Most methods today rely on seeds takenfrom a crystal formed by ammonothermal growth. Seeds thick enough foruse in ammonothermal growth that are produced by e.g. vapor phaseepitaxy typically crack, especially on a nitrogen-polar face of acrystal (such as the c-plane face). Consequently, while people may havetried obtaining seeds for ammonothermal crystal growth by forming theseeds in a faster-growth method, people have met with limited success inproducing seeds via a method in which crystals grow faster than in anammonothermal process.

This invention discloses group III nitride crystal which may be used forseed crystals in the ammonothermal bulk growth. In addition, thisinvention discloses methods of fabricating group III nitride crystals,which may be used for seed crystals in the ammonothermal bulk growth.Also this invention discloses a method of growing bulk crystals of groupIII nitride in supercritical ammonia using the group III nitridecrystals as seeds.

SUMMARY OF THE INVENTION

In one instance, the invention provides a wafer or other substrate ofgroup III nitride having a first, nitrogen-polar c-plane side and asecond side opposite to the first side. The first side has an exposednitrogen-polar face of single crystalline or highly orientedpolycrystalline group III nitride. The second side has either a groupIII polar, c-plane face of polycrystalline phase or amorphous phase ofgroup III nitride. The structural quality of the group III nitride ishighest on the first side and gradually degrades towards the secondside. The structural degradation from the first side to the second sidecan therefore be gradual and continual. With this structure, the firstside is free of crystal cracks.

In some instances, a wafer or crystal as disclosed herein has a first,nitrogen-polar face that is single-crystal group III nitride and asecond face that is oriented polycrystalline group III nitride,unoriented polycrystalline group III nitride, amorphous group IIInitride, or a mixture of these. In some other instances, a wafer orcrystal as disclosed herein has a first, nitrogen-polar face that isoriented polycrystalline group III nitride and a second face that isunoriented polycrystalline group III nitride, amorphous group IIInitride, or a mixture of these. In any event, the second face has poorerstructural quality than the first face of the wafer or crystal so thatthe wafer or crystal has sufficiently low stress within it that theresulting wafer or crystal does not crack on its first, nitrogen-polarface.

The invention also provides methods of fabricating group III nitridecrystal explained above. Using an epitaxial growth method such as HVPE,group III nitride crystal is grown on a substrate with group III polarface exposed. By changing growth condition such as temperature and/orambient oxygen concentration during the growth, the structural qualityis degraded gradually and preferably continually through growth. Thegrown group III nitride crystal on the substrate may split into twowafers upon cooling inside the epitaxial growth reactor, after coolinginside the epitaxial growth reactor, or outside of the epitaxial growthreactor. One of the split wafers has a first side exposing nitrogenpolar c-plane and a second side exposing group III polar c-plane,polycrystalline phase or amorphous phase of group III nitride.

In addition, the invention provides methods of growing bulk crystal ofgroup III nitride such as gallium nitride using the group III nitridecrystal explained above in supercritical ammonia.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings in which like reference numbers representcorresponding parts throughout:

FIG. 1 is a schematic drawing of the group III nitride crystal.

In the figure each number represents the followings:

1. A group III nitride crystal,

1A. A first side of the crystal exposing nitrogen polar c-plane surface,

1B. A second side opposite to the first side/

FIG. 2 is a schematic drawing of group III nitride and substratedepicted at steps A-E during fabrication of the group III nitridecrystal.

In the figure each number represents the followings:

2. A substrate,

3. A group III nitride grown on a substrate,

4. A group III nitride grown with changing growth parameter so that thestructural quality degrades gradually and continually along the growthdirection.

5. A location of separation that can occur upon or after cooling,

6. One of the split wafers which contains the substrate.

7. A group III nitride layer remaining on a substrate 2 after a groupIII nitride wafer 6 has separated from the substrate.

DETAILED DESCRIPTION OF THE INVENTION

Overview

The group III nitride crystal of the present invention is typically usedas a seed crystal for ammonothermal bulk growth. The group III nitrideis typically GaN although it can be any solid solution of group IIInitride expressed as Ga_(x)Al_(y)In_(1-x-y)N (0≦x≦1, 0≦x+y≦1). The groupIII nitride crystal has a first side exposing nitrogen polar surface ofc-plane and a second side exposing either group III polar (e.g. Ga polarin the case of GaN) surface of c-plane, polycrystalline phase, oramorphous phase of group III nitride. The group III nitride crystal hasa high structural-quality nitrogen polar first face and poorerstructural-quality second face. With this structure, one can eliminatecracks exposed on the first side of the crystal.

The structural quality of the first side is higher than that of thesecond side. “Structural quality” means how perfect the atomicarrangement is within the bulk crystal (the overall uniformity of thecrystal lattice's unit cell across a plane of the bulk crystal, wherethe plane is parallel to the surface of the substrate on which the bulkcrystal is grown), which can be evaluated with X-ray rocking curve orother analytical methods. If it is evaluated with X-ray rocking curve,the FWHM of the rocking curve of 002 reflection is smaller for the firstside than the second side. The characteristics of the group III nitridecrystal explained here may be suitable for usage as seed crystals in theammonothermal bulk growth.

To fabricate group III nitride crystals explained above, one can useepitaxial growth method such as HVPE. Other methods like metalorganicchemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), fluxmethod, high-pressure solution growth, sputtering can also be used aslong as the method is compatible with a heterogeneous substrate such assapphire, silicon carbide, silicon and gallium arsenide.

The structural quality is degraded along the growth direction bychanging growth conditions as more group III nitride is deposited on asubstrate. The growth conditions are changed to degrade structuralquality gradually (i.e. the unit cell is made less uniform or perfect innew growth as growth proceeds as shown by e.g. XRD results) so that thegroup III nitride has high-quality in a plane parallel to the surface ofthe substrate but poorer quality in a plane farther away from thesubstrate. The structural quality may be continually degraded from thehigh quality plane to the plane farther from the substrate. The changemay be e.g. linear, exponential, or other continuous function. Thestructural quality may alternatively be progressively changed, step bystep, preferably by small amounts. A sufficient amount of the group IIInitride is deposited on the substrate to form a wafer of group IIInitride that can be used in subsequent ammonothermal growth using thatwafer. The quality is degraded at a rate that provides high-qualitygroup III nitride at or near a separation point from the substrate(where the first face of the separated wafer will be) but sufficientlypoor quality at the second face that the bulk group III nitride of thewafer has low stress. The low amount of stress prevents the first,nitrogen-polar face from cracking when the wafer is separated from thesubstrate on which the wafer was grown but still provides a wafer whichis suitable for use in an ammonothermal method of growing high-qualitygroup III nitride on its first, nitrogen-polar face. The resultant groupIII nitride crystal is therefore one that has a high structural quality,nitrogen-polar first face and a poorer structural quality second facewith lower stress than a comparative crystal formed by maintainingcrystal growth conditions constant during crystal growth or adjustingconditions to improve structural quality of new growth. The resultantcrystal is also thick enough for use as a seed in a subsequentammonothermal growth of group III nitride on its first, nitrogen-polarface. Growth temperature and/or concentration of impurity such as oxygenin the ambient gas can be changed during growth to degrade crystalquality. For instance, growth temperature can be decreased during groupIII nitride growth to reduce crystal quality of the group III polarface. Alternatively or additionally, oxygen concentration in the ambientduring growth may be increased to reduce crystal quality of the groupIII polar face. The growth conditions can be changed continually todegrade crystal quality farther from the substrate. The change may bee.g. linear, exponential, or other continuous function, such as bydecreasing the temperature linearly or increasing oxygen concentrationin the ambient linearly. The change in growth conditions mayalternatively be progressively changed, step by step, preferably bysmall amounts. The resulting group III nitride substrate can thereforehave good crystal quality on a nitrogen-polar face, lower stress in thegroup III nitride, and poorer crystal quality at a group III polar faceof the substrate.

After growth on the heterogeneous substrate has finished, upon or aftercooling, the grown group III nitride crystal on the substrate maysometimes split into two pieces (we call this as self-separation)possibly due to the graded structure, difference in thermal expansionbetween the substrate and group III nitride, and/or difference inlattice constant between the substrate and group III nitride.Self-separation often occurs in new group III nitride that was depositedon group III nitride layer 7 of FIG. 2 when a heterogeneous substrate isused to make a crystal or wafer, such that group III nitride layer 7 isthicker than group III nitride layer 3 on substrate 2. Layer 7 in thisinstance may include group III nitride layer 3 that was originallydeposited on substrate 2 as well as a portion of new group III nitridegrown on layer 3. If self-separation does not occur one can use aconventional removal method such as grinding or laser lift-off to removethe substrate.

The group III crystal explained above is suitable as a seed forammonothermal bulk GaN growth. Using an ammonothermal method such as onedisclosed in the U.S. Utility Patent Application Ser. No. 61/058,910(now U.S. Pat. No. 8,236,237), bulk crystal of group III nitride may begrown on the group III crystal. Due to crack-free surface and higherstructural quality on the first side, bulk crystal grown on such seedsshow good crystal quality.

Technical Description of the Invention

The schematic of the group III nitride crystal in this invention ispresented in FIG. 1. The group III nitride crystal has a first side (1A)exposing nitrogen polar c-plane of group III nitride with a miscut angleless than +/−5 degree. The crystal has a second side (1B) opposite tothe first side which exposes either group III polar c-plane,polycrystalline phase or amorphous phase of group III nitride.

The first side is single crystalline or highly oriented polycrystallinegroup III nitride and the second side is single crystalline,polycrystalline, or amorphous group III nitride. The structural qualityon the first side is higher than that on the second side. Here thestructural quality means perfection of atomic arrangement in the crystaland is typically characterized with X-ray diffraction or otheranalytical methods. In the case of X-ray diffraction, FWHM of X-rayrocking curve from 002 reflection is recorded from both sides. The FWHMfrom the first side is smaller than that from the second side. The FWHMfrom the first side is typically smaller than 1000 arcsec, preferablyless than 500 arcsec, and more preferably less than 200 arcsec. The FWHMfrom the second side is typically more than 500 arcsec, preferably 1000arcsec. If the second surface is either polycrystalline phase oramorphous phase, one cannot detect 002 peak in the X-ray. This mean thatthe structural quality is poor. Even if other analytical methods areused, the crystal quality on the first side shows higher than that onthe second side. The change in the structural quality from the firstside to the second side is preferably gradual and may also beprogressive or continual.

The non-uniform structural quality along the c-axis direction helps toeliminate cracks on the first side because the low-quality crystalunderneath the first side acts as a buffer to reduce residual stress inthe crystal.

The thickness of the group III nitride crystal is typically more than0.1 mm to maintain its shape, preferably more than 0.3 mm, and morepreferably between 0.4 to 1 mm.

As explained later, the group III nitride crystal can be fabricated withan epitaxial growth method like HVPE. Other methods such as MOCVD, MBE,a flux method, high-pressure solution growth or sputtering can be usedas long as these methods are compatible with heterogeneous substratessuch as sapphire, silicon carbide, silicon and gallium arsenide.

High structural quality and crack-free characteristic on the first side(i.e. nitrogen polar c-plane surface) is beneficial to the ammonothermalbulk growth because a bulk group III nitride crystal is typically grownon the nitrogen polar c-plane in this method. When the group III nitridecrystal in this invention is used as a seed crystal in the ammonothermalgrowth, the first surface is preferably lapped and polished for highlevel of flatness and appropriate atomic arrangement on the surface. Thefirst side is optionally polished with chemical mechanical polishing toobtain atomically flat surface and remove subsurface damage caused bythe prior process. By covering the second side of the crystal withanother seed (the second side attached together so that the sides withpoor crystal quality face one another) or other masking material (suchas silver foil, nickel foil, vanadium foil or other metal foils),high-quality bulk crystal of group III nitride can be obtained on thefirst side of the group III nitride crystal.

To fabricate the group III nitride crystal explained above, an epitaxialgrowth of group III nitride is conducted on a substrate preferably withHVPE. The substrate may be heterogeneous substrate such as sapphire,silicon carbide, silicon or gallium arsenide, or homogeneous substratesuch as GaN, AlN, InN or their solid solutions. Other epitaxial growthmethod such as MOCVD, MBE, a flux method, high-pressure solution growthor sputtering can be used as long as these methods are compatible withheterogeneous substrates such as sapphire, silicon carbide, silicon andgallium arsenide.

FIG. 2 shows a particular sequence of process steps in a methodaccording to this invention. FIG. 2A shows a substrate 2 before growinggroup III nitride. In an epitaxial growth reactor such as HVPE reactor,single crystalline or highly oriented polycrystalline group III nitridelayer 3 is grown. The growth temperature is typically between 950 and1150° C. After this step, the growth condition such as temperature isgradually decreased and/or concentration of impurity in ambient gas isgradually increased to deteriorate the structural quality gradually toform the group III nitride crystal layer 4. The gradual change instructural quality is depicted by the crystal color darkening in thedirection of growth. After growth is finished, the group III nitridecrystal on the substrate is cooled. During or after the cooling processthe group III nitride crystal may crack along a horizontal line 5,resulting in splitting into two wafers. One wafer 6 contains thesubstrate and the other wafer is the group III nitride crystal in thisinvention. We call this splitting as self-separation.

If self-separation does not occur, one can remove the substrate withconventional methods such as mechanical grinding and laser lift-off.After removal of the substrate, one obtains a group III nitride crystalin this invention.

This group III nitride crystal can be used as a seed crystal in theammonothermal growth of bulk GaN. As disclosed in the U.S. UtilityPatent Application Ser. No. 61/058,910 (now U.S. Pat. No. 8,236,237),for example, seed crystals, mineralizer such as sodium metal, flowrestricting devices such a baffles, gallium containing nutrient such aspolycrystalline GaN and ammonia is loaded in a high-pressure reactor.The inside of the high-pressure reactor is divided into at least tworegions namely a seed region and a nutrient region. In the case ofammonobasic condition (i.e. using alkali metal or alkali earth metal asa mineralizer), the seed region is located below the nutrient region.The baffles separate these two regions. The high-pressure reactor isheated so that the appropriate temperature difference is made to growgroup III nitride.

In the case of ammonobasic growth of GaN, bulk GaN crystal typicallyshows better quality on nitrogen face than on gallium face. Therefore,the group III nitride crystal in this invention is beneficial to theammonothermal growth. By covering the group III polar side of thecrystal, we can grow high-quality GaN crystal selectively on thenitrogen polar side. There are a few ways of masking the group III polarface. One way is to attach two seeds together on group III polar sidesto make one hybrid seed exposing nitrogen polar surface on the bothsides. The other way is to mount the seed crystal on a metal plate suchas vanadium, nickel, silver, and nickel-chromium alloys with nitrogenpolar facing up. After the ammonothermal growth, bulk GaN crystal isobtained on the nitrogen side of the group III nitride crystal.

EXAMPLE 1 Growth Number 0858

GaN crystal was grown by HVPE. 2″ c-plane sapphire substrate having GaNlayer grown by MOCVD was loaded in an HVPE reactor. After ramping thesubstrate temperature to about 1000° C. under constant flow of ammoniaand nitrogen, gallium chloride gas was introduced to grow singlecrystalline GaN. After three hours of growth, the growth temperature wasgradually reduced over 13 hours. The temperature was reduced linearly by100° C. over 13 hours, resulting in a temperature reduction rate of 100°C. per 13 hours. After growing total 16 hours (3 hours of constanttemperature and 13 hours of graded temperature), the supply of galliumchloride was stopped and the furnace was turned off. At about 800° C.,the ammonia supply was stopped. The GaN crystal was cooled in thereactor until the temperature reaches about 300° C. When the crystal wastaken out of the reactor, the GaN crystal was self-separated from thesubstrate portion.

Since the substrate portion has a layer of GaN, the self-separationoccurred somewhere inside the GaN crystal. The thickness of c-planesapphire was 0.45 mm, the thickness of the portion containing thesapphire was 0.89 mm, the thickness of the GaN crystal separated fromthe substrate was 1.78 mm. The first side of the GaN crystal showedclear color whereas the second side of the GaN crystal showedgrayish/blackish color. The clear GaN crystal contains oxygen of lessthan about 10¹⁷ cm⁻³ whereas the grayish/blackish GaN contains oxygen ofmore than about 10¹⁹ cm⁻³. The X-ray measurement showed 002 peak fromthe first side (nitrogen polar side) but no peak from the second side.This means that the second side surface is covered with eitherpolycrystalline or amorphous GaN. The first side was free of cracks. Themiscut angle measured with X-ray rocking curve was within +/−5 degree.

EXAMPLE 2 Grinding/Lapping of the Crystal

Both sides of the GaN crystal were ground with a diamond grinder toobtain a GaN wafer having a thickness of 1.1 mm. The FWHM of X-rayrocking curve from the first side was 1382 arcsec whereas the secondside did not show a 002 peak. Then, both sides of the GaN crystal waferwere further ground and lapped with diamond slurry. The total thicknessbecame 0.85 mm with Ra roughness on the nitrogen side of 0.5˜0.8 nm andRa roughness on the gallium side of 0.8˜1.2 nm. The FWHM of the X-rayrocking curve from the first side improved to 1253 arcsec. The firstside did not have any crack.

EXAMPLE 3 Ammonothermal Growth Using the Obtained GaN Crystal

The GaN crystal wafer obtained in Example 2 was used as a seed crystalfor ammonothermal bulk growth. A high-pressure reactor was filled withthe seed, sodium metal, baffles, polycrystalline GaN nutrient andammonia. Then, the high-pressure reactor was tightly sealed and heatedto about 550° C. After 11 days of growth, a bulk GaN crystal havingthickness of about 2.07 mm was obtained. The FWHM of the X-ray rockingcurve from the first side improved to 1048 arcsec. The crystal also didnot have crack.

EXAMPLE 4 Growth Number 0895

Similar to example 1, a GaN crystal was grown by HVPE. 2″ A c-planesapphire substrate having a GaN layer grown by MOCVD was loaded in aHVPE reactor. Although the growth conditions and duration were the sameas Example 1, the substrate and new crystal did not completely separate.The thickness excluding the sapphire substrate portion was 2.63 mm. TheFWHM of the X-ray rocking curve from 002 reflection on the first sidewas about 925 arcsec, whereas the FWHM on the second side was 1580arcsec. In this case the Ga polar side had low-quality crystalline GaN(i.e. highly oriented polycrystalline) on the exposed second face. Theresidual sapphire substrate was removed with a diamond grinder, and thenboth sides of the GaN crystal were also ground to obtain a wafer of 0.44mm thickness.

Advantages and Improvements

The group III nitride crystal of this invention has higher structuralquality on the nitrogen polar surface and is free of cracks. Suchcrystal is suitable for a seed crystal in the ammonothermal bulk growth.A method of fabricating the group III nitride crystal in this inventionuses epitaxial growth of group III nitride on a substrate, followed byself-separation or removal of the substrate. By changing growthconditions gradually during crystal growth, the structural quality isdeteriorated gradually, thus avoiding crack generation on the first sideof the crystal. By using such crystals for ammonothermal bulk growth,one can obtain high-quality bulk crystals of group III nitride such asGaN.

Possible Modifications

Although the example describes crystals of GaN, similar benefit of thisinvention can be expected for other group III nitride alloys of variouscompositions, such as AlN, AlGaN, InN, InGaN, or GaAlInN.

Although the preferred embodiment describes HVPE as an epitaxial growthmethod, other methods such as MOCVD, MBE, a flux method, high-pressuresolution growth or sputtering can be used as long as they are compatiblewith heterogeneous substrates.

Although the preferred embodiment describes a seed crystal having adiameter of 2″, similar benefit of this invention is expected for alarger diameter such as 4″, 6″ and larger.

Although the preferred embodiment describes X-ray characterization ofstructural quality, other methods such as Rutherford backscattering(RBS), reflection high-energy electron diffraction (RHEED), transmissionelectron microscopy (TEM) can be used to evaluate the structural qualityof the surfaces.

Although the example describes a diamond grinding to remove the sapphiresubstrate, laser lift-off or other methods can be used to remove thesubstrate.

It is not necessary to start the method using a heterogeneous substratethat has a group III nitride material deposited upon it. One can startwith a substrate and immediately begin forming group III nitride uponthat substrate under conditions that produce high structural-qualitygroup III nitride and then adjust deposition conditions to graduallyform poorer structural-quality group III nitride as further group IIInitride deposition occurs. Further, it is not necessary to start with aheterogeneous substrate. One may utilize a group III nitride substrateand grow on a group III polar face of the substrate, forminghigh-quality group III nitride, and then change growth conditions togradually form poorer-quality group III nitride in subsequent growth.

Following are various examples of processes, machines, articles ofmanufacture, and/or compositions of matter that illustrate certainembodiments of but do not limit the scope of the claimed invention:

1. A group III nitride crystal comprising,

-   -   (a) a first side having an exposed nitrogen polar c-plane        surface with miscut angle less than +/−5 degree.    -   (b) a second side opposite to the first side having an exposed        group III polar c-plane surface, polycrystalline phase or        amorphous phase of the group III nitride, wherein crystal        structural quality of the first side is better than crystal        structural quality of the second side.

2. A group III nitride crystal according to paragraph 1, wherein thesurface of the first side is free from cracking.

3. A group III nitride crystal according to paragraph 1 or paragraph 2,wherein crystal quality degrades gradually from the first side to thesecond side of the group III nitride crystal.

4. A group III nitride crystal according to any one of paragraphs 1through 3, wherein the oxygen concentration on the first side is smallerthan the oxygen concentration of the second side.

5. A group III nitride crystal according to paragraph 4 wherein theoxygen concentration of the second side is more than ten times higherthan the oxygen concentration of the first side.

6. A group III nitride crystal according to any one of paragraphs 1through 5, wherein the full width half maximum of X-ray rocking curve ofthe 002 reflection from the first side is smaller than the full widthhalf maximum of X-ray rocking curve of the 002 reflection from thesecond side.

7. A group III nitride crystal according to paragraph 6 wherein the FWHMof the X-ray rocking curve of the 002 reflection from the first side isless than 1000 arcsec.

8. A group III nitride crystal according to paragraph 7 wherein the FWHMof the X-ray rocking curve of the 002 reflection from the first side isless than 500 arcsec.

9. A group III nitride crystal according to paragraph 8 wherein the FWHMof the X-ray rocking curve of the 002 reflection from the second side isgreater than 500 arcsec.

10. A group III nitride crystal according to any one of paragraphs 6-8wherein the FWHM of the X-ray rocking curve of the 002 reflection fromthe second side is greater than 1000 arcsec.

11. A group III nitride crystal according to any one of paragraphs 1through 6, wherein the thickness of the crystal is more than 0.1 mm.

12. A group III nitride crystal according to paragraph 11 wherein thecrystal is more than 0.5 mm thick.

13. A group III nitride crystal according to paragraph 11 wherein thecrystal is more than 1 mm thick.

14. A group III nitride crystal according to any one of paragraphs 1through 13, wherein the first side is polished sufficiently that thefirst side is suitable for ammonothermal growth of a bulk crystal.

15. A group III nitride crystal according to any one of paragraphs 1through 14, wherein the crystal is fabricated by hydride vapor phaseepitaxy.

16. A group III nitride crystal according to any one of paragraphs 1through 15, wherein the transition of crystal quality from the firstside to the second side is continuous.

17. A group III nitride crystal according to any one of paragraphs 1through 16, wherein the group III nitride is GaN.

18. A group III nitride crystal according to any one of paragraphs 1through 17 wherein the crystal has no cracks throughout the crystal.

19. A group III nitride wafer of a crystal of any one of paragraphs 1through 18.

20. A group III nitride wafer according to paragraph 19 wherein thewafer is a single crystal group III nitride wafer.

21. A method of fabricating a group III nitride crystal comprising;

-   -   (a) growing a single crystalline or a highly oriented        polycrystalline group III nitride layer on a substrate, wherein        the exposed surface of the layer is group III polar c-plane;    -   (b) further growing single crystalline or highly oriented        polycrystalline group III nitride having gradually degraded        crystal structure such that the exposed surface of the crystal        becomes a group III polar c-plane, polycrystalline phase or        amorphous phase;    -   (c) removing the substrate to obtain a crystal having a first        nitrogen polar c-plane surface and a second group III polar        c-plane surface, polycrystalline phase or amorphous phase of the        group III nitride.

22. A method of fabricating a group III nitride crystal according toparagraph 21, wherein steps (a) and (b) are conducted with hydride vaporphase epitaxy.

23. A method of fabricating a group III nitride crystal according toparagraph 21 or 22, wherein the substrate is a heterogeneous substrate.

24. A method of fabricating a group III nitride crystal according to anyone of paragraphs 21 through 23, wherein the step (c) comprisesself-separation of the substrate upon or after cooling.

25. A method of fabricating a group III nitride crystal according to anyone of paragraphs 21 through 23 wherein the step (c) comprises grindingof the substrate.

26. A method of fabricating a group III nitride crystal according to anyone of paragraphs 21 through 23 wherein the step (c) comprises laserlift-off of the substrate.

27. A method of fabricating a group III nitride crystal according to anyone of paragraphs 21 through 26, wherein the step (b) is conducted atlower temperature than that of step (a).

28. A method of fabricating a group III nitride crystal according toparagraph 27, wherein the temperature in the step (b) is graduallydecreased during growth.

29. A method of fabricating a group III nitride crystal according toparagraph 28, wherein the temperature in the step (b) is decreasedlinearly during growth.

30. A method of fabricating a group III nitride crystal according toaccording to any one of paragraphs 21 through 29, wherein the step (b)is conducted at higher oxygen concentration than that of step (a).

31. A method of fabricating a group III nitride crystal according toparagraph 30, wherein the oxygen concentration in the step (b) isgradually increased during growth.

32. A method of fabricating a group III nitride crystal according toparagraph 31, wherein the oxygen concentration is increased linearly.

33. A method of fabricating a group III nitride crystal according to anyone of paragraphs 21 through 32, further comprising the following steps;

-   -   (a) grinding the second surface;    -   (b) grinding the first surface;    -   (c) lapping the first surface.

34. A method of fabricating a group III nitride crystal according to anyone of paragraphs 21 through 33, wherein the group III nitride is GaN.

35. A group III nitride crystal formed by a method of any one ofparagraphs 21 through 34.

36. A method of fabricating a bulk crystal of group III nitride insupercritical ammonia using a group III nitride crystal according to anyone of paragraphs 1 through 19 and 35 as a seed crystal in thesupercritical ammonia.

37. A method according to any one of paragraphs 21 through 34 and 36,wherein the group III nitride is GaN.

38. A method of fabricating a bulk crystal of gallium nitride in ahigh-pressure reactor comprising

-   -   (a) placing at least one gallium nitride seed crystal in the        high pressure reactor;    -   (b) placing at least one kind of mineralizer in the high        pressure reactor;    -   (c) placing at least one flow-restricting plate in the high        pressure reactor    -   (d) placing gallium containing nutrient in the high pressure        reactor;    -   (e) placing ammonia in the high pressure reactor;    -   (f) sealing the high pressure reactor;    -   (g) heating the high pressure reactor with appropriate        temperature difference between the region for the seed crystals        and the region for the nutrient; wherein the crystal structural        quality of the nitrogen polar surface of the gallium nitride        seed crystal is better than the crystal structural quality of        the gallium polar surface of the seed crystal.

39. A method of fabricating a bulk crystal of gallium nitride accordingto paragraph 38, wherein the nitrogen polar surface of the galliumnitride seed crystal is free from cracking.

40. A method of fabricating a bulk crystal of gallium nitride accordingto paragraph 38 or paragraph 39, wherein the oxygen concentration of thenitrogen polar surface of the gallium nitride seed crystal is less thanthe oxygen concentration of the gallium polar surface.

41. A method of fabricating a bulk crystal of gallium nitride accordingto paragraph 40 wherein the oxygen concentration of the gallium polarsurface is more than ten times higher than the oxygen concentration ofthe opposite side.

42. A method of fabricating a bulk crystal of gallium nitride accordingto paragraph 38 and 39, wherein the full width half maximum of X-rayrocking curve of the 002 reflection from the nitrogen polar surface ofthe gallium nitride seed crystal is smaller than the full width halfmaximum of X-ray rocking curve of the 002 reflection from the oppositeside.

43. A method of fabricating a bulk crystal of gallium nitride accordingto any one of paragraphs 38 through 42, wherein the thickness of thegallium nitride seed crystal is more than 0.1 mm.

44. A method according to paragraph 43 wherein the thickness of thegallium nitride seed crystal is at least 0.5 mm.

45. A method of fabricating a bulk crystal of gallium nitride accordingto any one of paragraphs 38 through 43, wherein the nitrogen polarsurface of the gallium nitride seed crystal is polished to obtain asuitable surface for ammonothermal growth of bulk crystal.

46. A method of fabricating a bulk crystal of gallium nitride accordingto any one of paragraphs 38 through 45, wherein the gallium nitride seedcrystal is fabricated by hydride vapor phase epitaxy.

47. A method of fabricating a bulk crystal of gallium nitride accordingto any one of paragraphs 38 through 46, wherein the transition ofcrystal quality from the nitrogen polar surface of the gallium nitrideseed crystal to the opposite side is gradual.

Variations on these and other embodiments as disclosed herein arerecognizable by one skilled in the art, and these variations are alsowithin the scope of the invention disclosed herein. Consequently, theclaims are to be accorded a broad interpretation, consistent with thedisclosure of the new technology and principles disclosed herein.

What is claimed is:
 1. A method of fabricating a group III nitridecrystal comprising; (a) growing a single crystalline or a highlyoriented polycrystalline group III nitride layer on a substrate, whereinthe exposed surface of the layer is group III polar c plane; (b) furthergrowing single crystalline or highly oriented polycrystalline group IIInitride having gradually degraded crystal structure such that theexposed surface of the crystal becomes a group III polar c-plane,polycrystalline phase or amorphous phase; (c) removing the substrate toobtain a crystal having a first nitrogen polar c-plane surface and asecond group III polar c-plane surface, polycrystalline phase oramorphous phase of the group III nitride.
 2. A method of fabricating agroup III nitride crystal according to claim 1, wherein the step (a) and(b) are conducted with hydride vapor phase epitaxy.
 3. A method offabricating a group III nitride crystal according to claim 2, whereinthe substrate is a heterogeneous substrate.
 4. A method of fabricating agroup III nitride crystal according to claim 1, wherein the substrate isa heterogeneous substrate.
 5. A method of fabricating a group IIInitride crystal according to claim 1, wherein the step (c) comprisesself-separation of the substrate upon or after cooling.
 6. A method offabricating a group III nitride crystal according to claim 1, whereinthe step (c) comprises grinding of the substrate.
 7. A method offabricating a group III nitride crystal according to claim 1, whereinthe step (c) comprises laser lift-off of the substrate.
 8. A method offabricating a group III nitride crystal according to claim 1, whereinthe step (b) is conducted at lower temperature than that of step (a). 9.A method of fabricating a group III nitride crystal according to claim8, wherein the temperature in the step (b) is gradually decreased duringgrowth.
 10. A method of fabricating a group III nitride crystalaccording to claim 9, wherein the temperature in the step (b) isdecreased linearly during growth.
 11. A method of fabricating a groupIII nitride crystal according to claim 2, wherein the step (b) isconducted at lower temperature than that of step (a).
 12. A method offabricating a group III nitride crystal according to claim 11, whereinthe temperature in the step (b) is gradually decreased during growth.13. A method of fabricating a group III nitride crystal according toclaim 12, wherein the temperature in the step (b) is decreased linearlyduring growth.
 14. A method of fabricating a group III nitride crystalaccording to claim 3, wherein the step (b) is conducted at lowertemperature than that of step (a).
 15. A method of fabricating a groupIII nitride crystal according to claim 14, wherein the temperature inthe step (b) is gradually decreased during growth.
 16. A method offabricating a group III nitride crystal according to claim 15, whereinthe temperature in the step (b) is decreased linearly during growth. 17.A method of fabricating a group III nitride crystal according to claim1, wherein the step (b) is conducted at higher oxygen concentration thanthat of step (a).
 18. A method of fabricating a group III nitridecrystal according to claim 8, wherein the oxygen concentration in thestep (b) is gradually increased during growth.
 19. A method offabricating a group III nitride crystal according to claim 1, furthercomprising the following steps; (a) grinding the second surface; (b)grinding the first surface; (c) lapping the first surface.
 20. A methodof fabricating a group III nitride crystal according to claim 1, whereinthe group III nitride is GaN.
 21. A group III nitride crystal formed bya method of claim
 1. 22. A method of fabricating a bulk crystal of groupIII nitride in supercritical ammonia comprising using a group IIInitride crystal according to claim 21 as a seed crystal in thesupercritical ammonia, and growing group III nitride upon the seedcrystal in the supercritical ammonia.
 23. A method according to claim 1,wherein the group III nitride is GaN.